The term “ionic liquids” is generally understood to mean salts or mixtures of salts whose melting point is below 100° C. (P. Wasserscheid, W. Keim, Angew. Chem. (2001), 112, 3926). Salts of this type known from the literature consist of anions, such as halostannates, haloaluminates, hexafluorophosphates or tetrafluoroborates combined with substituted ammonium, phosphonium, pyridinium or imidazolium cations. Several publications have already described the use of ionic liquids as solvents for chemical reactions (T. Welton, Chem. Rev. (1999), 99, 2071, P. Wasserscheid, W. Keim, Angew. Chem., (2000), 112, 3926). For example, hydrogenation reactions of olefins with rhodium(I) (P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Polyhedron 15/7, 1996, 1217-1219), ruthenium(II) and cobalt(II) complexes (P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Inorganica Chimica Acta 255, 1997, 207-209) have been carried out successfully in ionic liquids with tetrafluoroborate anion. The hydroformylation of functionalized and non-functionalized olefins is also possible with rhodium catalysts in ionic liquids with weakly coordinating anions (e.g. PF6, BF4) (Y. Chauvin, L. Mussmann, H. Olivier, European Patent, EP 776880, 1997; Y. Chauvin, L. Mussmann, H. Olivier, Angew. Chem., Int. Ed Engl., 1995, 34, 2698; W. Keim, D. Vogt, H. Waffenschmidt, P. Wasserscheid, J. of Cat., 1999, 186, 481).
Further important fields of application of ionic liquids consist of their use as extraction agents for material separation (J. G. Huddleston, H. D. Willauer, R. P. Swatlowski, A. E. Visser, R. D. Rogers, Chem. Commum. (1998), 1765-1766; b) A. E. Visser, R. P. Swatlowski, R. D. Rogers, Green Chemistry (2000), 2(1), 14) and of their use as heat carriers (M. L. Mutch, J. S. Wilkes, Proceedings of the Eleventh International Symposium on Molten Salts, P. C. Trulove, H. C. De Long, G. R. Stafford and S. Deki (Editors), Proceedings Volume 98-11, The Electrochemical Society, Inc, Pennington, N.J.; 1998, page 254).
Even if the definition of an ionic liquid includes those salts whose melting point is between room temperature and 100° C., it is still necessary and desirable for many applications for the ionic liquids to be liquid at temperatures below room temperature already.
Further, for all applications in which ionic liquids are used as solvents or solvent additives in the field of chemical synthesis or catalysis, but also as heat carriers or as extraction solvents, the use of very low viscosity ionic liquids is of high technical value. The lower the viscosity of the ionic liquids, the faster diffusion and mass transport processes occur in those applications. In most applications, this has direct consequences for the space-time yield that can be achieved, the energy requirements or the necessary amount of ionic liquids. To conclude, the economic efficiency of almost all applications of ionic liquids is essentially determined by their viscosity: the lower the viscosity of the ionic liquid employed, the greater the economic efficiency of a corresponding application.
Numerous examples of ionic liquids are known that are liquid at room temperature. However, as a rule these systems possess halide ions such as F−, Cl−, Br− or I− or those anions which contain halogen atoms, e.g. organohalides. Typical representatives of the latter anions include, by way of example and without limitation, (BF4), (PF6), (CF3CO2), (CF3SO3), ((CF3SO2)2N)−, (AlCl4)−, (Al2Cl7)− or (SnCl3)−. The use of such anions containing halogen atoms imposes serious restrictions on the applicability of the corresponding ionic liquids:                a) The use of these anions leads to considerable costs since even the alkali salts of these ions are very expensive;        b) The hydrolysis products of these anions containing halogen atoms lead to considerable corrosion in steel reactors and in some instances also in glass reactors;        c) The thermal disposal of a “spent” ionic liquid with anions containing halogen atoms usually causes corrosion and environmental problems and is therefore very costly. Disposal via degradation in a biological clarification plant is also rendered difficult by the presence of anions containing halogen atoms.        
In general, ionic liquids that are substantially free from halogen atoms (halogenic anions or organohalides) are therefore of particular interest, especially if they additionally possess the following properties:                a) a melting point and/or glass transition point of less than 25° C.;        b) a low viscosity (<0.8 Pas at 20° C. (800 cPs at 20° C.));        c) hydrolysis-stable in neutral aqueous solution (pH=7) up to 80° C.        
Among the ionic liquids free from halogen atoms according to the state of the art, there have been no representatives so far capable of satisfying this complex technical requirement profile. Thus, nitrate melts, nitrite melts, sulfate melts (J. S. Wilkes, M. J. Zaworotko, J. Chem. Soc. Chem. Commun. (1992), 965) and benzenesulfonate melts (H. Waffenschmidt, Dissertation, RWTH Aachen 2000) are known, however, these ionic liquids have melting points above room temperature. Hydrogen sulfates and hydrogen phosphates react in aqueous solution while splitting off one or several protons and form acidic aqueous solutions. Methyl sulfate and ethyl sulfate melts exhibit a distinct hydrolysis after only 1 h at 80° C. in aqueous solution with the formation of hydrogen sulfate anions and the corresponding alcohol (compare also Comparative Examples 1 and 2). Ionic liquids of general formula (cation) (R—O—SO3) have a viscosity higher than that required for use in most technical applications (see requirement b) if R merely represents a linear or branched, saturated or unsaturated, aliphatic or alicyclic alkyl group having 3-36 carbon atoms non-functionalized or functionalized with one or more groups Y, in which Y is an —OH, —OR″, —COOH, —COOR″, —NR2, —SO4, —F, —Cl, —Br, —I or —CN group, R″ representing a branched or linear hydrocarbon chain with 1-12 carbon atoms (see Comparative Example 3).
Therefore, it is the object of the present invention to provide halogen-free ionic liquids that have a melting point or glass point of 25° C. or lower, a viscosity of 0.8 Pas at 20° C. (800 cPs at 20° C.) or lower, and improved aqueous hydrolytic stability.